Have you heard of HIIT (high intensity interval training)? To put it briefly, you alternate very high intensity bursts of activity (e.g. sprinting full out) with short rest periods. This kind of training has shown to improve endurance greater than steady submaximal workouts (e.g. jogging for 60 minutes at the same pace). I love this workout because you can accomplish this in even just 10 minutes! I have never been a great endurance athlete, likely because my training as a dancer was essentially interval training (you do a dance full-out for 3-4 minutes then you rest before doing it all over again). Next time you head to gym swap your usual routine with a HIIT workout and see the world of a difference that occurs!